OFDM schemes are utilized as standards for IEEE 802.11a, ETSI BRAN'S HYPERLAN 2, European digital audio broadcasting (DAB) and digital TV DVB-T. A conventional single carrier transmission scheme in which information is carried by a single carrier causes interference between symbols to increase, so distortion also increases. Accordingly, an equalizer of a receiver must be complicated. In order to solve these problems of the conventional single carrier transmission scheme, OFDM schemes have been introduced.
OFDM schemes enable data to be transmitted using multi-carriers. Such OFDM schemes are able to convert data symbols input in series into parallel data symbols, to modulate each of the parallel symbols into a plurality of tone signals which are orthogonal to each other, and to transmit the modulated signals.
OFDM schemes have been widely applied to digital transmission technologies, such as digital audio broadcasting (DAB), digital television, wireless local area network (WLAN) or wireless asynchronous transfer mode (WATM). In particular, OFDM schemes maintain orthogonality between tone signals, unlike conventional multicarrier schemes, so it is possible to obtain optimum transmission efficiency during high speed data transmission. Additionally, almost the whole available frequency band can be utilized and multi-path fading can be reduced.
However, OFDM schemes have the disadvantage that OFDM signals exhibit a high Peak-to-Average Power Ratio (PAPR) due to modulation between multi-carriers. The PAPR is essentially identical to a Peak-to-Average Ratio (PAR). Since data is transmitted using multi-carriers in OFDM schemes, the final OFDM signal has an amplitude equal to the sum of amplitudes of individual carriers so that variation in the amplitude increases. Additionally, if phases of the individual carriers are identical, a very large value may be obtained. Accordingly, the signal is out of the linear operating range of a high power linear amplifier, so distortion may occur during a linear amplifying operation.
Therefore, methods for reducing such a PAPR have been studied. Among the methods, a tone reservation method has been provided, in which a Peak Reduction Kernel is carried and transmitted by a reserved tone, which is not used to transmit data, among a plurality of tones for generating multi carrier signals, so as to compensate a PAPR.
In more detail, some of the tones are reserved in a frequency domain. After an initial value (for example, 0) is temporarily carried by the reserved tones, the reserved tones are converted into time-domain signals, and a signal corresponding to a position having power greater than the permissible peak power is searched. A Peak Reduction Kernel for compensating the position is then carried by the reserved tones, so that the PAPR can be compensated.
However, some of the reserved tones may be used for other purposes, for example pilot transmission. Additionally, the position of reserved tones used for other purposes may change according to a predetermined pattern.
The Peak Reduction Kernels has to be carried by reserved tones other than the reserved tones used for other purposes (hereinafter, referred to as additional data tones), so it is difficult to detect an optimum Peak Reduction Kernel. Additionally, the position of reserved tones into which Peak Reduction Kernels are to be inserted changes in various ways according to the type of symbols, and accordingly it is not easy to determine a Peak Reduction Kernel according to the above change.
Therefore, there is a limitation to form a Peak Reduction Kernel, and the PAPR reduction efficiency may thus be reduced.